In gas turbine engines (see FIG. 1) for use in powering aircraft, air is directed through multiple stage compressors as it flows axially or axially and radially through the engine to a burner. As the air passes through each successive compressor stage, the pressure of the air is increased. Under certain conditions, such as when the engine is throttled back or during start up, the amount of air required in the burner is less than that flowing through the compressor. In this condition an engine surge or blow-out may occur, endangering the operation of the engine and the associated aircraft.
To mitigate against these conditions, such gas turbine engines have incorporated bleed valves in the engine casing forward of the burner which, when an engine surge is imminent, open to reduce airflow to the burner. These bleed valves have taken many forms from simple ports in the compressor casing which open via a movable valve clement to devices which separate adjacent segments of the engine casing thereby creating an opening. there between
One prior art bleed valve employing a moveable segment is depicted in FIGS. 2, 3 and 4. This bleed valve is operated by applying a tangential force derived from pressurized engine fuel, via a rod 6, to a linkage 8 connected to a movable segment 10 of the engine casing 11. The force moves the movable segment 10 in a helical direction such that the movable segment rotates tangentially about the air flow as well as moving the segment 10 forward toward the engine inlet 13. As the moveable segment 10 moves away from the stationary segment 12, an opening 14 is created between the moveable segment 10 and the stationary segment 12, permitting the pressurized air to escape, thereby lowering the air pressure in that portion of the compressor stage and consequently the pressure in the air reaching the burner. The relative position of the two segments along the axis is maintained by the linkage 8 as well as two other linkages 16 spaced about the outside of the stationary segment 12. These linkages comprise a flat metal connector 18 having two ends, a first end 20 being affixed to the outside surface of the stationary segment 12 via a pin 22 and mount 24 while the second end 26 is connected to the outside surface of the moveable segment 10 via a second pin 28 attached to a second mount 30 on the outer surface of the moveable segment 10. The linkages 16 connecting the stationary segment 12 with the movable segment 10 maintain the relative position of the two segments along the axis during operation of the bleed valve. This is important as the clearances inside the engine are limited and damage could occur if the segments were permitted to move outside their relative positions. To further assist in maintaining the position of the two segments during operation maintenance pads 32 are used. These comprise an L-shaped overhang or arm 34 affixed to the moveable segment 10 such that the distal end 36 of the overhang lies below the lip 38 of the stationary segment 12 and is in contact with a tab 40 positioned on the outside surface of the stationary segment 12 just below the lip 38 of the stationary segment. The end of the over hang 36 has a covering 46 of a friction reducing material to reduce the friction between the tab and the over hang as the moveable segment rotates during opening and closing. To insure that the seal is indeed sealed, coaxial mating lands and grooves 48 are formed in both the mating surfaces 42 and 44 which, when the valve is closed, improve the seal between the two surfaces.
During a potential surge condition or other condition, when the bleed valve must be open, the valve must respond quickly and without hesitation. Although the prior art design is adequate, with the limited force available from the fuel pressure to actuate the valve and the hot, dirty environment in which these valves operate, improvements are constantly being sought to lower the three required to operate the valves and to prevent fouling due to contamination which may slow the response of the valves. In addition, the mechanical linkages used to control the motion of the moveable segment permit more than the optimum amount of lateral motion desired. Further, the tabs and overhangs fixed to the bleed valve segments add weight and machining operations to the construction of the valve which translates into additional manufacturing costs. Therefore, what is needed in this art is an improved system to open and close the bleed valve of this design which would reduce the force required to open the valve and improve on the relative motion of the moveable segment and be less costly to manufacture.